The present application relates to a method and apparatus for real time passive estimation of chromatic dispersion in active wavelength division multiplexed optical network spans.
Optical fiber systems have become widely used for high capacity telecommunications networks, where data is typically transmitted as a stream of light pulses within an optical spectrum covering some range of optical frequencies around a central frequency. Such a stream of pulses is known as a xe2x80x9cchannelxe2x80x9d. The capacity of optical fiber communications systems has been increased both by increasing the data rate for each channel, and by multiplexing channels at different wavelengths onto a single optical fiber (known in the art as wavelength division multiplexing, or WDM). Future optical fiber networks are also expected to be xe2x80x9cagilexe2x80x9d, with the capability of adding and dropping optical channels at intermediate nodes in a network, and dynamically reconfiguring the optical paths through the network taken by each channel, with out converting optical signals to the electrical domain. These advanced networks require careful management of the distortions to optical signal pulses caused by propagation through optical fibers.
An optical pulse propagates through an optical system at a velocity known as the group velocity. The time delay for a pulse to propagate through an optical system is known as the group delay. In an optical fiber, the group velocity varies with wavelength, such that the longer wavelength components of an optical pulse propagate slightly faster or slower (depending on the sign of the chromatic dispersion) than the shorter wavelength components. This typically leads to a broadening in time of an optical pulse propagating through an optical fiber. This broadening is known as chromatic dispersion. As the pulses broaden, they eventually overlap in time, and can no longer be distinguished at an optical receiver. Chromatic dispersion is an inherent property of the optical fiber and is a function of link distance and wavelength. Thus, chromatic dispersion represents one of the fundamental limitations to the maximum data rates and transmission distances which can be achieved in an optical fiber network.
As data rates increase, such as 10 or 40 Gb/s, and channel densities increase, chromatic dispersion must be more closely managed. xe2x80x9cDispersion limited distancexe2x80x9dis a metric used to describe the distance where the theoretical bit error rate (BER) reaches 1 dB. This distance is inversely proportional to the square of the data rate, or 4 times increase in data rate results in {fraction (1/16)} the distance without dispersion compensation.
In an all optical domain network, the residual dispersion will become dynamic and dependent on the physical path of the optical signal, requiring a dynamic dispersion compensation device. Also in multispan systems a cost effective way to track the residual dispersion from span to span is needed, as this residual dispersion is not constant across the assembly of optical carriers. Either case requires a way to monitor the residual dispersion profile for trends or changes which indicate the potential for a future outage before a failure occurs.
Known dispersion compensators, typically lengths of fiber which possess a high dispersion slope with the opposite sign to the fiber network, or tunable dispersion compensators based on Bragg gratings or etalons, are set up on a span basis at commissioning using specialized test equipment. Currently there is no way to monitor the residual chromatic dispersion in real time during operation.
Examples of etalon based dispersion compensators are disclosed in U.S. Pat. Nos. 5,557,468; and 6,519,065 and U.S. publication No. 2002/0176659, all commonly owned by the present assignee. Additional tunable dispersion compensators are disclosed in U.S. Pat. No. 6,501,874 to a tunable Bragg grating dispersion compensator, for example, and U.S. publication No. 2002/0186438.
Eye diagrams have been observed in many data transmission systems as indicative of system problems or increasing error rates. U.S. Pat. No. 5,333,147 discloses the use of an automated monitoring system to monitor error rates in eye diagrams as indications of stress or other problems in a RF transmission system. A pseudo-error region within the ideal eye diagram is designated, and incursions into this region are counted in order to give indication of eye closing, before actual failure occurs.
U.S. Pat. No. 5,774,242 discloses a system for measuring distortion in optical transmission using an eye mask. In this case, very simple maximum and minimum threshold values are established, as well as a phase window of acceptable phase jitter. This simple eye mask gives an indication of signal distortions including chromatic dispersion, noise, interference, etc, from amplifiers and other devices in the fiber link, environmental factors and from the fiber itself.
The system disclosed by U.S. Pat. No. 5,774,242, however, is not adapted for use in a DWM optical network. Furthermore, the simplicity of the eye mask does not provide information for measuring the effective BER, or distinguishable data for determining the sign of the accumulated dispersion. Thus, feedback to a dispersion compensator produces a slower corrective response.
For high data rate systems, an accurate dispersion monitor is still needed.
The present invention has found that chromatic dispersion can be effectively estimated from careful contour mapping of the eye diagram of a signal, in order to provide advanced indication of the effective BER for dynamic chromatic dispersion compensation. A dispersion compensation monitor for DWM optical systems capable of creating and analyzing the contour map of the eye diagram is further disclosed.
Accordingly, the present invention provides a method of monitoring chromatic dispersion of an optical signal of at least one optical transmission channel in an optical transmission system comprising the steps of:
tapping a portion of the optical signal;
determining metric values from an eye diagram of the tapped optical signal comprising the steps of:
receiving the portion of the optical signal at a linear optical receiver and converting the optical signal to an electrical signal;
recovering a clock signal from the electrical signal;
sampling the signal amplitude at the center of the eye diagram and comparing the amplitude to a threshold value to determine a bit value;
sampling the signal amplitude for at least four points in the eye diagram by adjusting the phase of the clock signal in accordance with a predetermined mapping algorithm in order to calculate a contoured eye map for selected metrics;
computing metric values from the contoured eye map; and,
correlating the computed metric values with a predetermined relationship between dispersion and the selected metrics, to determine the dispersion value.
In a further embodiments of the invention, a chromatic dispersion monitor for monitoring accumulated chromatic dispersion in an optical signal comprising a bit stream within a span of active transmission of an optical network comprises:
a coupler for tapping a portion of the optical signal;
a linear optical receiver for detecting the portion of the optical signal and for converting the detected portion of the optical signal into an electrical signal;
an electrical clock recovery circuit for recovering the clock from the electrical signal;
a reference detector for sampling an amplitude of a signal at the center of a signal bit and comparing the detected value to a threshold value for determining a bit value;
a variable time delay circuit for adjusting a phase of the clock relative to the bit stream controlled by a mapping algorithm to sample more than three amplitude values at different phase delays to define a contoured eye map of the optical signal;
a probe circuit for determining a bit value for each of the more than three sampled amplitude values in comparison to associated thresholds in the mapping algorithm;
a comparator circuit for comparing the bit value of the reference detector to the bit value of the probe detector for generating an estimated bit error rate;
a processor for calculating metric values from the contoured eye map and correlating the calculated metric values to a stored library of chromatic dispersion in relation to predefined metric values.